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In the 1980s, climate scientists in Russia and the U.S. theorized that
all-out nuclear war between the superpowers would result in a "nuclear
winter," as smoke from the atomic explosions blackened the sky and sent
summer temperatures plummeting below freezing  killing crops and eventually
starving all those who survived the initial explosions. Now that the risks
of an all-out U.S.-Russian exchange have diminished, scientists are
looking at the climactic effects of regional nuclear war  and the predictions are
still sobering.

Alan Robock, a Professor in the
Department of Environmental Sciences at Rutgers University who participated
in the original nuclear winter research, recently completed a study on the
results of a nuclear war between India and Pakistan. He spoke with TIME from
his office in New Brunswick, New Jersey. (See pictures from the aftermath of the Mumbai terror attacks.)

Tensions between India and Pakistan have been high recently. If they escalated to all-out nuclear war, what would be the effect to the
global climate?

We looked at a scenario in which each country used 50 Hiroshima-sized
weapons, which they are believed to have in their arsenals. That's enough
firepower to kill around 20 million people on the ground. We were surprised
that the amount of smoke produced by these explosions would block out
sunlight, cool the planet, and produce climate change unprecedented in
recorded human history.

Your study predicts mass cooling. With all the heat and
radioactivity of the explosions, why wouldn't nuclear war warm the planet?

It has nothing to do with the radioactivity of the explosions  although that
would be devastating to nearby populations. The explosions would set off
massive fires, which would produce plumes of black smoke. The sun would heat
the smoke and lift it into the stratosphere  that's the layer above the
troposphere, where we live  where there is no rain to clear it out. It would
be blown across the globe and block the sun. The effect would not be a
nuclear winter, but it would be colder than the little ice age [in the 17th
and 18th centuries] and the change would happen very rapidly  over the course
of a few weeks.

Would you be able to see the smoke?

The sky would not be blue. It would be grey.

And what would the results be for humanity?

We calculated that there would be a shortening of the growing season in the
mid-latitudes  that includes Europe and America in the Northern Hemisphere  by
a couple of weeks. The growing season is defined as the period between the last frost in spring and
first frost in the fall. Some crops that need the whole growing season would
not reach fruition and there would be no yield. Others would grow more
slowly and produce a small yield. In addition there would be less
precipitation and it would be darker, also damaging yield. You compound that
with [the shutdown of] the current global network of food trading  countries would likely stop
shipping food and focus on feeding their own populations  and it's a big
crisis. We don't have the resources to do detailed analyses on the impacts
of crops in different farming regimes but this suggests it could be a very
serious problem.

How confident are you that your modeling is correct?

We used ModelE, designed by NASA's Goddard Institute for Space Studies, and
one of the models used to produce the results of the Intergovernmental Panel on Climate Change (IPCC). The model
does an excellent job of simulating climate change that resulted from
volcanic eruptions in the past. That gave us confidence. What's more, a
group repeated the calculations for the Pakistan-India scenario with a
different model at the National Center for Atmospheric Research in Boulder,
Colo., and the results almost exactly agreed. Their research showed how the
smoke from the fires would open up holes in the ozone, which would cause
even more problems for humanity. We'd like other people to test the
calculations with their models, but we're pretty confident that they'll get
the same answer.

So we get a clue of the climatic effects of nuclear war from volcanic
eruptions?

Yes. 1816 was known as the "year without summer." It followed the Tambora
Volcano eruption in Indonesia in 1815. It was sudden climate change on a
similar scale, and it resulted in a severe famine in Europe, food riots and
mass emigrations. Volcanic aerosols have a lifetime of about a year in the
stratosphere. The lifetime of soot from nuclear fires is about five years.
It's obviously much harder for a society to recover from such an extended
cooling.

Some scientists, most notably Freeman Dyson of The Institute for
Advanced Study in Princeton, have stirred controversy by arguing that
nuclear weapons are a more urgent environmental threat than global warming.
Do you agree?

Yes. If India and Pakistan engaged in nuclear war, they would use about 0.3%
of the global nuclear stockpile. And still the effects on the
climate would be dramatic. Our calculations on nuclear winter from the early
1980s have been confirmed by modern climate models. And fundamentally the
situation hasn't changed  even with reduced stockpiles there still exists
enough weapons to cause nuclear winter. That's something that maybe people
don't realize.

I think we have to solve the problem of the existence of all these weapons
before we have the luxury of worrying about global warming.